Electrostatic discharge (ESD) is a serious problem for many types of solid state electronics, such as integrated circuits (ICs). Electronic components such as ICs can be exposed to ESD from various different sources, such as the human body, assembly equipment, or basic packaging materials. Contact between the sources and a grounded IC can generate large enough currents through the IC to significantly damage its internal circuitry.
The effects of ESD create special problems with touch electronics, i.e., electronics intended for touching by the body. For example, electronic fingerprint sensors allow a user to swipe or press a finger over some portion of the circuit in order to read the user's fingerprint. It would be impractical or inconvenient for a user to have to ground his or her body prior to touching the sensor in order to dissipate an electrostatic charge.
Conventional fingerprint sensors include a silicon chip with an exposed surface for receiving human touch. These fingerprint sensors can be easily damaged physically or mechanically because of the exposed surface, reducing the durability and/or reliability of the sensor. The conventional fingerprint sensors as well as newer, more advanced “flexible” fingerprint sensors, which enable a user to swipe a finger across a polyimide surface without directly contacting the sensor circuitry, are both susceptible to ESD damage. For example, electrostatic charge can build up on the polyimide surface of the flexible fingerprint sensor as a user swipes his or her finger. This charge can continue to increase in potential until the path of least resistance is found and the charge dissipated. In certain cases, the charge can discharge to the sensor circuitry, causing damage to sensitive electronic components such as IC input/output cells.
The current ESD protection used in the fingerprint sensor industry uses a metal ring surrounding the perimeter of the sensor. This arrangement requires an additional metal layer in the sensor manufacture, thus increasing the cost of the sensor. The inventions disclosed herein teach a new kind of ESD protection for touch electronics that reduces the manufacture cost and increase the durability of the electronics.